1. Field of the Invention
The present invention relates to a decalcification method of an aqueous solution comprising multivalent cations Ca2+ and Mg2+ and anions able to form complexes with at least a part of said multivalent cations, such as phosphate, lactate or citrate anions.
2. Description of the Related Art
In the dairy industry, as in many other industries, the presence of calcium and/or magnesium in liquids to be treated restricts some operations and particularly the operations of concentration of these liquids.
Thus, for instance, in the case of the production of crystallized lactose from whey, the presence of calcium interferes with the concentration of this whey and limits the quality of the lactose produced due to a co-precipitation of calcium salt.
The presence of calcium and/or magnesium also restrains the use of the separation methods used for the purification, such as the electrodialysis or the chromatography.
Several techniques have been used in the past to eliminate the Ca2+ and Mg2+ ions contained in an aqueous medium.
It is in particular a question of techniques using strong cationic resins of which the counter-ion is Na+ or K+, for the decalcification (softening) of water or juice, in particular in sugar refinery.
The elimination of the Ca2+ and Mg2+ ions makes it possible to improve the performances of the methods down-stream, by limiting risks of precipitation of insoluble salts.
During the water or juice percolation through these strong. cationic resins, the Ca2+ and Mg2+ ions contained in this water or juice, are exchanged with Na+ or K+ ions of the resins.
When these latter are saturated, they are regenerated by the passing through them a NaCl aqueous solution or an aqueous solution containing Na+ or K+ ions. There is then exchange of the Na+ or K+ ions of this solution with the Ca2+ and Mg2+ ions which attached to the resins.
It will however be noted that in the case of wheys for instance, and in particular sweet wheys, the decalcification by means of cationic resins of which the counter-ion is Na+ or K+, following the cycle known as softening cycle, becomes difficult due to the formation of complexes between the Ca2+ and Mg2+ ions and the anionic portion of some acids, generally weak, of which the most well known are phosphoric acid and some organic acids such as citric acid and lactic acid.
The Ca2+ and Mg2+ ions thus complexed are therefore much less available for an exchange with the Na+ or K+ ions of the resins and the decalcification yields are therefore reduced.
In order to get round this difficulty, we had re-course to weak cationic resins, known as chelating, having more affinity with the Ca2+ and Mg2+ ions than the aforementioned strong cationic resins.
However, when the counter-ion of these resins is Na+ or K+, their regeneration is costly, because they require a first regeneration with an acid, generally hydrochloric acid or sulfuric acid, to replace by H+ ions the Ca2+ and Mg2+ ions attached by these resins, then a second regeneration with soda or potash to replace the H+ ions by Na+ or K+ ions.
The total demineralization of wheys is sometimes contemplated by passage in series at first through a cationic resin of which the counter-ion is H+, which may be regenerated with an acid, then through an anionic resin of which the counter-ion is OH−, which may be regenerated with a base.
During the percolation of wheys through the cationic resin, the Ca2+ and Mg2+ ions replace the H+ ions attached to the resin. It results in a substantial drop of the pH of the wheys in treatment, drop which has for effect to destroy the aforementioned complexes between the Ca2+ and Mg2+ ions and the phosphate anions and/or the organic acid anions (lactate, citrate, etc.) contained in said wheys. These Ca2+ and Mg2+ ions are then available for the ion exchange.
If with such a method, an almost pure lactose solution can actually be produced, this method is nevertheless costly in chemicals and produces important volumes of effluents. Moreover, this technique is not very selective and eliminates in not very differentiated manner all the ion species whatever their impact is on the methods downstream.